The principal components of a prior art KrF excimer laser chambers are shown in FIG. 1. This chamber is a part of a laser system used as a light source for integrated circuit lithography. These components include a chamber housing 2. The housing contains two electrodes cathode 84 and anode 83 each about 55 cm long and spaced apart by about 20 mm, a blower 4 for circulating a laser gas between the electrodes at velocities fast enough to clear (from a discharge region between the two electrodes) debris from one pulse prior to the next succeeding pulse at a pulse repetition rate in the range of 1000 Hz or greater. (Gas velocities of about 10 m/s for each 1000 Hz pulse rate is typical.) The chamber includes a water cooled finned heat exchanger 6 for removing heat added to the laser gas by the fan and by electric discharges between the electrodes. Blower 4 is typically a squirrel cage type tangential fan providing high gas flow but at relatively low differential pressure. The chamber may also include baffles 60 and 64 and vanes 66 and 68 for improving reducing discharge caused acoustic effects and the aerodynamic geometry of the chamber. The laser gas is comprised of a mixture of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. Each pulse is produced by applying a very high voltage potential across the electrodes with a pulse power supply which causes a discharge between the electrodes lasting about 30 nanoseconds to produce a gain region about 20 mm high, 3 mm wide and 525 mm long. (Two capacitors of a peaking capacitor bank are shown at 62.) The discharge deposits about 2.5 J of energy into the gain region. As shown in FIG. 2, lasing is produced in a resonant cavity, defined by an output coupler 20 and a grating based line narrowing unit (called a line narrowing package or LNP, shown disproportionately large) 22 comprising a three prism beam expander, a tuning mirror and a grating disposed in a Littrow configuration. The energy of the output pulse 3 in this prior art KrF lithography laser is typically about 10 mJ.
FIG. 3 shows an enlarged view of cathode 84 and anode 83. Each is about 3 cm wide but the discharge region 85 is only about 3 to 4 mm wide. The direction of gas flow is shown at 86 and a gas flow of 20 m/s is indicated. The cathode and anode are typically brass. The cathode is typically slidingly mounted on an insulator 84a and the anode is typically mounted on a metal support 83A.
These KrF lithography lasers typically operate in bursts of pulses at pulse rates of about 1000 to 2000 Hz. Each burst consists of a number of pulses, for example, about 80 pulses, one burst illuminating a single die section on a wafer with the bursts separated by down times of a fraction of a second while the lithography machine shifts the illumination between die sections. There is another down time of a few seconds when a new wafer is loaded. Therefore, in production, for example, a 2000 Hz, KrF excimer laser may operate at a duty factor of about 30 percent. The operation is 24 hours per day, seven days per week, 52 weeks per year. A laser operating at 2000 Hz xe2x80x9caround the clockxe2x80x9d at a 30 percent duty factor will accumulate more than 1.5 billion pulses per month. Any disruption of production can be extremely expensive. For these reasons, prior art excimer lasers designed for the lithography industry are modular so that maintenance down time is minimized.
Maintaining high quality of the laser beam produced by these lasers is very important because the lithography systems in which these laser light sources are used are currently required to produce integrated circuits with features smaller than 0.25 microns and feature sizes get smaller each year. Laser beam specifications limit the variation in individual pulse energy, the variation of the integrated energy of series of pulses, the variation of the laser wavelength and the magnitude of the bandwidth of the laser beam.
Typical operation of electric discharge laser chambers such as that depicted in FIG. 1 causes electrode erosion. Erosion of these electrodes affects the shape of the discharge which in turn affects the quality of the output beam as well as the laser efficiency. Typically, anode erosion in these excimer lasers is two to three times as severe as cathode erosion. Electrode erosion is the result of a complex combination of physical phenomena including fluorine chemical attack and ion induced sputter. Use of alloys of copper for electrodes for gas discharge lasers is well known. For example, a common electrode material is a brass known as C36000 which is comprised of 61.5% copper, 35.5% zinc and 3% lead. It is known to anneal brass parts before they have been machined to make the parts less brittle.
ArF excimer lasers are very similar to KrF excimer lasers except that the laser gas comprises argon, neon and fluorine. The effects of electrode erosion are known to be more severe in ArF lasers than in KrF lasers, primarily because ArF lasers are more sensitive to loss mechanisms.
Spinodal copper alloys are well-known copper alloys, first studied in the early 1930""s. The process of spinodal decomposition hardens the copper alloys by creating regions of periodically varying concentrations of the alloy components without creating precipitates. Spinodal decomposition is reported to increase both hardness and ductility of copper alloys. A spinodal copper alloy (designated as C72900 ASTM B740-84) which has a composition of 77% copper, 15% nickel and 8% tin is commercially available and is widely used for bushings, bearings, springs and electronic connectors. It combines high yield strength and formability with good stress relaxation, electrical conductivity and corrosion resistance. The spinodal properties are produced by annealing the alloy at temperatures between about 200xc2x0 C. and about 520xc2x0 C. FIG. 5 is a TTT diagram extracted from a paper by Zhao and Notis, Acta metall., Vol. 46, No. 12, pp. 4203-4218, 1998. These materials are known to be good choices for sleeve bearings. Spinodal copper alloys are available from suppliers such as Anchor Bronze and Metal with offices in Bay Village, Ohio.
What is needed is a gas discharge laser having electrodes with reduced erosion rates.
The present invention provides electrodes comprised of spinodal copper alloys. Applicant""s tests have shown erosion rates of these alloys under certain environmental conditions are a factor of 5 or more lower than erosion rates of similar prior art copper alloys. In one application, the erosion of spinodal electrodes was at least an order of magnitude lower than the prior art material. A preferred application of these electrodes are as electrodes in excimer lasers which utilize a circulating laser gas containing fluorine. A preferred spinodal copper alloy is a copper-tin-nickel alloy known as spinodal bronze. These alloys are prepared using spinodal decomposition. This material forms atomic layers several atoms thick. The spinodal decomposition process permits atoms of one kind to concentrate to an extent while maintaining a relatively uniform crystal structure. A specific alloy of spinodal bronze commercially available which has been tested by Applicant with amazing results is comprised primarily of about 80 percent copper, about 7 percent tin and about 12.5 percent nickel. It is commercially available and sold under the tradename, Nicomet(copyright)3 Spinodal Bronze.